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March 1, 2011 -- Adolescents and young adults who smoke marijuana have an increased risk for experiencing psychotic symptoms, according to a new study.

The new findings appear online in the journal BMJ.

Researchers assessed marijuana use during a 10-year study of 1,923 participants aged 14 to 24 in Germany.

Those participants who had no psychotic symptoms and had never tried marijuana when the study began and then started using marijuana had nearly double the risk of experiencing psychotic symptoms in the future.

And those who used marijuana before start of the study and who continued use over the study period had an increased risk of persistent psychotic symptoms, the study shows.

“Our study confirmed cannabis as an environmental risk factor, impacting on the risk of psychosis by increasing the risk of incident psychotic experiences and if use continues over time, increasing the risk of persistent psychotic experiences,” write researchers who were led by Jim van Os, MD, PhD, a professor of psychiatry and neuropsychology at the South Limburg Mental Health Research and Teaching Network of Maastricht University Medical Center in the Netherlands.

Marijuana Use and Psychosis

Exactly how marijuana may affect risk of developing psychotic symptoms and in whom is not fully understood, but the new study seems to debunk the thinking that people with psychotic symptoms turn to marijuana as a means of self-medication, because marijuana use in the study preceded psychotic symptoms.

“People who reported new psychotic symptoms, and who persisted in using cannabis ... reported more persistent psychotic symptoms than those who stopped using cannabis,” write Wayne Hall, PhD, of the University of Queensland in Herston, Australia, and Louisa Degenhardt, PhD, of the Burnet Institute in Melbourne, Australia, in an accompanying editorial.

“In the light of these findings and those of earlier studies, it is likely that cannabis use precipitates schizophrenia in people who are vulnerable because of a personal or family history of schizophrenia,” they write.

Marijuana and Psychosis: Which Comes First?

“Carefully conducted prospective studies such as this one provide the best evidence available that cannabis contributes to the cause of some cases of schizophrenia,” says Matthew Large of Prince of Wales Hospital in New South Wales, Australia. Large recently published a similar study linking marijuana use to earlier age of onset of schizophrenia.

“The study goes a long way to clarifying that it is not psychosis that causes cannabis use, but rather the reverse,” he says in an email.

Tomas Silber, MD, an adolescent medicine doctor at the Children’s National Medical Center in Washington, D.C., reviewed the new study for WebMD. He says the new study helps to clarify the relationship between marijuana use and psychotic episodes.

“Marijuana use increases these episodes,” he says.

“Many people have suspected that people smoke marijuana as a way of treating themselves, and this study disproves that connection,” he says. “It is not a two-way street, it’s a one-way street.”